Redox flow secondary batteries are to store and discharge electricity, and belong to large-size stationary batteries used for leveling the amounts of electricity used.
The redox flow secondary battery is configured such that a positive electrode and an electrolyte solution comprising a positive electrode active substance (positive electrode cell) and a negative electrode and a negative electrode electrolyte solution comprising a negative electrode active substance (negative electrode cell) are separated by a diaphragm; charge and discharge are carried out by utilizing the oxidation and reduction reactions of both the active substances; and the electrolyte solutions including both the active substances are circulated from storage tanks to an electrolytic bath, and a current is taken out and utilized.
As an active substance contained in an electrolyte solution, there are used, for example, iron-chromium-based ones, chromium-bromine-based ones, zinc-bromine-based ones, and vanadium-based ones utilizing the difference in electric charge.
Particularly, vanadium-type secondary batteries, since having advantages of a high electromotive force, a high electrode reaction rate of vanadium ions, only a small amount of hydrogen generated as a side-reaction, a high output, and the like, are being developed earnestly.
For diaphragms, devices are made so that electrolyte solutions comprising active substances of both electrodes are not mixed. However, conventional diaphragms are liable to be oxidized and for example a problem thereof is that the electric resistance needs to be made sufficiently low.
Although in order to raise the current efficiency of batteries, the permeation of each active substance ion contained in the cell electrolyte solutions of both the electrodes (contamination with electrolytes in electrolyte solutions of both electrodes) is demanded to be prevented as much as possible, an ion-exchange membrane excellent in the ion permselectivity, in which protons (H+) carrying the charge easily sufficiently permeate, is demanded.
The vanadium-type secondary battery utilizes an oxidation and reduction reaction of divalent vanadium (V2+)/trivalent vanadium (V3+) in a negative electrode cell, and oxidation and reduction reaction of tetravalent vanadium (V4+)/pentavalent vanadium (V5+) in a positive electrode cell. Therefore, since electrolyte solutions of the positive electrode cell and the negative electrode cell have ion species of the same metal, even if the electrolyte solutions are permeated through a diaphragm and mixed, the ion species are normally reproduced by charging; therefore, there hardly arises a large problem as compared with other metal species. However, since active substances becoming useless increase and the current efficiency decreases, it is preferable that active substance ions permeate freely as little as possible through the diaphragm.
There are conventionally batteries utilizing various types of diaphragms (in the present description, referred to as “electrolyte membrane” or simply “membrane” in some cases); and for example, batteries are reported which use porous membranes allowing free permeation by an ionic differential pressure and an osmotic pressure of electrolyte solutions as the driving force. For example, Patent Literature 1 discloses a polytetrafluoroethylene (hereinafter, also referred to as “PTFE”) porous membrane, a polyolefin (hereinafter, also referred to as “PO”)-based porous membrane, a PO-based nonwoven fabric, and the like as a diaphragm for a redox battery.
Patent Literature 2 discloses a composite membrane in combination of a porous membrane and a hydrous polymer for the purpose of the improvement of the charge and discharge energy efficiency of a redox flow secondary battery and the improvement of the mechanical strength of a diaphragm thereof.
Patent Literature 3 discloses a technology in which a membrane of a cellulose or an ethylene-vinyl alcohol copolymer is utilized as a nonporous hydrophilic polymer membrane excellent in the ion permeability and having a hydrophilic hydroxyl group for the purpose of the improvement of the charge and discharge energy efficiency of a redox flow secondary battery.
Patent Literature 4 states that the utilization of a polysulfone-based membrane (anion-exchange membrane) as a hydrocarbon-based ion-exchange resin makes the current efficiency of a vanadium redox secondary battery 80% to 88.5% and the radical oxidation resistance excellent.
Patent Literature 5 discloses a method of raising the reaction efficiency by making expensive platinum to be carried on a porous carbon of a positive electrode in order to raise the current efficiency of a redox flow secondary battery, and describes a Nafion (registered trademark) N117 made by Du Pont K.K. and a polysulfone-based ion-exchange membrane as a diaphragm in Examples.
Patent Literature 6 discloses an iron-chromium-type redox flow battery in which a hydrophilic resin is coated on pores of a porous membrane of a polypropylene (hereinafter, also referred to as “PP”) or the like. An Example of Patent Literature 6 discloses a membrane covered in a thickness of several micrometers with a fluorine-based ion-exchange resin (made by Du Pont K.K., registered trademark: “Nafion”) on both surfaces of a PP porous membrane of 100 μm in thickness. Here, Nafion is a copolymer comprising a repeating unit represented by —(CF2—CF2)— and a repeating unit represented by —(CF2—CF(—O—(CF2CFXO)n—(CF2)m—SO3H))— wherein X=CF3, n=1, and m=2.
Patent Literature 7 discloses an example of a vanadium-type redox flow secondary battery decreased in the cell electric resistance as much as possible and raised in the efficiency by the improvement of the electrodes including the usage of a two-layer liquid-permeable porous carbon electrode having a specific surface grating.
Patent Literature 8 discloses an example of a vanadium-type redox flow battery using an anion-exchange type diaphragm having a low membrane resistance, being excellent in the proton permeability and the like, and being composed of a crosslinked polymer having a pyridinium group and utilizing N+ as a cation. The crosslinked polymer disclosed is a polymer obtained by copolymerizing a pyridinium group-comprising vinyl polymerizable monomer, a styrene-based monomer and the like, and a crosslinking agent such as divinylbenzene.
Patent Literature 9, for the purpose of reducing the cell resistance and improving the power efficiency and the like, discloses a redox flow secondary battery utilizing a membrane as a diaphragm, the membrane having a structure in which a cation-exchange membrane (a fluorine-based polymer or another hydrocarbon-based polymer) and an anion-exchange membrane (a polysulfone-based polymer or the like) are alternately laminated, and having a cation-exchange membrane disposed on the side of the membrane contacting with a positive electrode electrolyte solution.
Patent Literature 10 discloses a secondary battery using as a diaphragm a membrane excellent in the chemical resistance, low in the electric resistance, and excellent in the ion permselectivity, which is an anion-exchange membrane made by compositing a porous base material composed of a porous PTFE-based resin with a crosslinked polymer having a repeating unit of a vinyl heterocyclic compound having two or more hydrophilic groups (vinylpyrrolidone having an amino group, or the like).
The principle described therein is that although metal cations, having a large ion diameter and a much amount of electric charge, receive an electric repulsion by cations of a diaphragm surface layer part and are inhibited from the membrane permeation under the potential difference application, protons (H−), having a small ion diameter and being monovalent can easily diffuse and permeate in the diaphragm having cations to thereby give a low electric resistance.
Patent Literature 11 discloses examples using Nafion (registered trademark of Du Pont K.K.) and Gore Select (registered trademark of W. L. Gore & Associates, Inc.).